Apparent translatory flow in groundwater recharge and runoff generation

A sound understanding of solute transport under stormflow conditions is crucial for assessing groundwater and stream water contamination risk. The vadoze zone exhibits its maximum protective effect, when solute transport occurs via translatory flow. In contrast, short-term hydraulic short circuits via preferential flow can have considerable harmful effects on water quality. The Lehstenbach study combines comprehensive physical and hydrochemical measurements that allow improved understanding of the short-term stream discharge and groundwater recharge dynamics. The data set covers the 1998 catchment wetting-up period, including the second to highest discharge peak, since measurements began in 1987. During that storm, the pressure wave reached 0.9 m depth within 2 h, preceding the discharge peak by another 2 h. In contrast, shallow groundwater response at 3 m depth was delayed considerably. Soil hydrometric data and temperature, aluminum, sulfate, and dissolved organic carbon dynamics in stream water and groundwater indicated translatory flow during groundwater recharge and stormflow runoff generation. In contrast, the observed decline in silica concentration of groundwater and stream water provided strong evidence that seepage flux was restricted to a small fraction of the total soil water pool. Exchange with the matrix was limited by the slow kinetics of silica dissolution, while sulfate and aluminum kinetics are quite rapid, and this feature explains the apparent discrepancy between silica, sulfate, and aluminum data. The results emphasize that preferential flow phenomena are not so much due to inherent properties of the soil matrix as depending on the scale of observation and the observed parameters and their kinetics of equilibrating with the matrix during subsurface transport. (C) 2002 Elsevier Science B.V. All rights reserved.